1. Field of the Invention
The present invention relates generally to optical metrology systems and, more particularly, to sample point interferometry methods and apparatus for measuring simultaneously changes in figure of a segmented primary mirror and spacing between primary and secondary mirrors.
2. Description of the Prior Art
Large aperture spaceborne optical systems will be needed to meet 21st century requirements for both scientific and military applications. However, conventional telescope design approaches employing large, monolithic, rigid mirrors can be excessively heavy. Furthermore, sensitivity to thermal and mechanical disturbances increases with aperture size. Active optical system approaches offer the potential of meeting these new requirements with less weight, and with the ability to actively correct for the effects of thermal and mechanical disturbances. Active system approaches include the use of segmented, and/or deformable primary mirrors, as well as secondary and tertiary mirrors that are adjustable in rigid body degrees-of-freedom.
The most general design approach for the largest system element, the primary mirror, is to segment the mirror, each segment being individually deformable by means of an array of figure control actuators. However, this design approach implies an ability to accurately and rapidly measure the primary mirror's figure so that it can be actively controlled. This entails measuring the figure of each segment as well as the relative tilt and piston (phasing) errors between segments. Alignment of secondary and tertiary mirrors requires the measurement of all rigid body degrees-of-freedom (tilt, piston, decenter and roll).
Many different figure and rigid body sensing concepts have been proposed. However, using on-board semiconductor laser diodes as a light source for multiwavelength interferometry, the sample point interferometer (SPI) has become a preferred instrument to measure figure and rigid body degrees-of-freedom simultaneously. The technique is non-contact, has a large dynamic range, and can measure at high bandwidths.
The original Sample Point Interferometer patent (Montagnino, U.S. Pat. No. 4,022,532) shows a reference beam that is local to the interferometer, whether it is located at the secondary mirror or the focal plane. This configuration can monitor the primary mirror, but laser frequency stability requirements can be very tight. The disclosure of this U.S. Patent is incorporated by reference herein in its entirety.
A more recent design that has been implemented utilizes a "remote" reference beam having a path length approximating the pathlengths of the sample beams which address the primary mirror. This configuration can reduce laser frequency stability requirements by an order of magnitude. However, this configuration is not appropriate for measuring primary/secondary mirror spacing.
In the inventor's commonly assigned U.S. Pat. No. 5,220,406, an improved system is disclosed, although it is similar in some respects to that disclosed in the aforementioned U.S. Pat. No. 4,022,532. Generally, the system includes an interferometer, a number of light reflecting spots placed at sample points on a surface to be monitored, and a source of light for generating a reference beam of collimated light and a measuring beam of collimated light. For the SPI type described therein, the light source is a laser or other source of monochromatic light. The path length of the reference beam may either be fixed, or temporally modulated. The measuring beam is directed through focussing and/or deflecting optics which defines a field that includes the reflective spots. Light reflected back through the optics from the spots is combined with the reference beam and applied to a detector that includes a plurality of light intensity detecting elements. The detector is positioned in relation to the focussing optics so that a conjugate image of the field of reflective spots is formed at the operative surface of the detector. The separate elements of the detector are positioned to detect light reflected back from the reflecting spots, combined with the reference beam light.
The configuration of the surface is monitored by comparing the relative intensities of the light derived from the sample points. When the reference beam path length is modulated, the configuration of the surface is monitored by comparing the phase relationships of the variations in the intensity of light derived from the interference of light from the sample points with light from the modulated reference beam. The system is initially set so that the intensity of signals from each of the detector elements are in a preestablished phase relation with the reference beam path length modulation. Any change thereafter in the phase relationships indicates distortion, i.e., movement forward or back from predetermined relative positions of the respective sample points. A change in phase indicates the direction and amount of distortion. This information may be utilized for applying force either manually or by an automatic system, at indicated points, so to adjust the surface to a desired configuration.